In mobile telephone technology, a broad-band amplifier, which can be used to amplify several mobile telephone signals at the same time, is increasingly used in order to economise on costs. For instance, two or more W-CDMA (Wideband—Code Division Multiple Access) signals positioned on different frequency bands, or a GSM signal and a W-CDMA signal may be required at the same time. In order to test a power amplifier in a base station, for example, this power amplifier must be supplied with a measurement signal, which simulates the operational situation as close to reality as possible, i.e. with a combined signal derived from the two above-named individual signals. For this purpose, the individual signals have, hitherto, been generated in separate signal generators as separate high-frequency signals, which are then combined at the output of the signal generators, i.e. at the high-frequency side. This raises the problem of synchronising the two separate signal generators. Moreover, it is disadvantageous that several signal generators are needed, because it is difficult to operate several separate devices, and the devices require complex wiring at the output side.
Regarding background art, reference is made, by way of example, to U.S. Pat. No. 5,465,050, wherein the output signal from a signal generator is combined on the high-frequency side, via a resistor network, with the output signal from a noise generator.